Optical storage technology comprises a wide and growing variety of disc and application specifications. Disc specifications include, for example, CD-ROM and DVD-ROM for pre-recorded discs, CD-R, DVD-R, and DVD+R for write-once discs, and CD-RW, DVD-RW, DVD-RAM, and DVD+RW for rewritable discs. The disc format specifications generally define the physical characteristics of the disc (e.g., mechanical properties, optical signal characteristics, physical arrangement, writing methods, and testing conditions). Application specifications include DVD-Video for video content, DVD-Audio for audio content, and DVD-VR and DVD+VR for real-time video recording (e.g., in camcorders and personal video recorders [PVRs]).
In many optical disc specifications, an optical disc may comprise two areas, including a user data area and a disc information (lead-in) area. The user data area is generally used to write application data, including video, audio, information tables, file system data, etc. The disc information (lead-in) area generally includes data such as disc size, disc type, disc layout, etc. In some optical disc specifications (e.g., CD-R, DVD-R and DVD+R), the disc generally can be written only once. In other disc specifications or formats (e.g., CD-R/W, DVD+RW), data may be written to the disc more than once.
Optical storage media generally store data as a sequence (e.g., a continuous track) of “pits” (or “marks”) and “lands” (or “space”) on a data-bearing surface that is made reflective by the application of a metallic layer during manufacture. The “lands” are generally parts of the track that are not pits. Pits in read-only storage media are generally molded into the data bearing surface when the discs are formed. Recordable and re-writable discs are generally produced as blanks, and have only a preformed groove or “pre-groove” (together with a limited amount of embossed data in most cases) included during molding. Data is stored on recordable or re-writable optical storage media using the same pit-land principle, however the pits are generally added by “burning” a special phase-change material layer applied to the disc substrate. In order to write to a recordable or re-writable disc, an optical pick-up head of an optical disc is generally equipped with a higher power write laser in addition to the read laser. Alternatively, one laser can generally perform both functions by operating at lower power output for read operations, and a range of higher power outputs for write operations.
The amount of power used to burn pit features is critical to the shape of those features. The geometry of the shape of the pit features affects the read-back performance of read-back systems. Recordable and re-writable optical storage media are produced by a wide variety of vendors using different materials. Thus, an appropriate writing power level for one medium may be too high or too low for another medium, even if both media are manufactured in accordance with the same specification. Therefore, most optical storage medium specifications include some facility for a vendor to specify an optimal power level. However, even the vendor specified power level may be suboptimal due to normal process variations in the medium and/or the recording device or operational variations in the recording device. Therefore, most optical storage medium specifications also define one or more areas of the media for performing optimal power calibration (OPC) by writing data at various power levels and read back the data to determine an optimal writing power level. The amount of space available for OPC operations may be limited, and the OPC operations may need to be run multiple times if data is written to the medium in multiple sessions and/or on using multiple different recording devices on the same medium. Consequently, it is desirable for optical storage medium recording devices to obtain the most accurate calibration data using as little space in the OPC area as possible.
In optical storage media, everyday handling damage, such as dust, fingerprints, and tiny scratches, may affect retrieved data and disrupt the functionality of a recording or playback device. Specific sequences of pits and lands are particularly susceptible to defects in the medium, and playability can be improved if such sequences are barred from recording. Various encoding methods are used in optical media to avoid this problem. For example, run length limited (RLL) codes are generally used, wherein the spectrum (power density function) of the encoded sequence vanishes at the low-frequency end, and both the minimum and maximum number of consecutive bits of the same kind are within specified bounds (or constraints).
In the compact disc (CD) standard, data is encoded using eight-to-fourteen modulation (EFM). Under EFM rules, the data to be stored is first broken into 8-bit blocks (bytes). Each 8-bit block is translated into a corresponding 14-bit codeword using a lookup table. The 14-bit codewords are chosen such that binary ones are always separated by a minimum of two binary zeroes and a maximum of ten binary zeroes. This is because a binary one is stored on the disc as a change from a land to a pit or a pit to a land, while a binary zero is indicated by no change. Because EFM ensures there are at least two zeroes between every two ones, it also ensures that every sequence of adjacent pit and land features is at least three clock cycles long. This reduces the demands on the optical pickup used in the playback mechanism. The ten consecutive-zero maximum ensures worst-case clock recovery in the player. EFM generally requires 3 merging bits between adjacent 14-bit codewords to ensure that consecutive codewords do not violate the specified minimum and maximum run-length constraint. Thus, 17 bits of disk space are generally needed to encode 8 bits of data.
Referring now to FIG. 1, three binary sequences are shown. Binary sequence 101 represents the number 136 (e.g., “10001000” in an eight-bit binary code). Binary sequence 102 represents an EFM code for the same byte of data (e.g., “01001001000001”). Binary sequence 103 represents the same EFM code, except that a “1” in binary sequence 102 is represented in binary sequence 103 as a transition between a “1” and a “0” or between a “0” and a “1,” corresponding to the pit and land transitions to be written to disc (e.g., “011100011111110”). It will be recognized that, using the encoding of FIG. 1C under the EFM constraints, there will never be a valid sequence of fewer than three “1”s or “0”s (after accounting for the merging bits before and/or after the EFM codeword). It will also be recognized that, while this encoding method is very valuable for avoiding errors in the optical storage medium, it also contains some overhead and/or redundant (e.g., non-information) data.
Similar run length limited codes are specified for other optical media. For example, DVDs are generally encoded using EFMPlus, which requires fewer packing bits than EFM encoding. The Blu-ray Disc (BD) and HD-DVD standards also specify encoding methods to be used on those media.
In order to perform OPC, pattern data (e.g., EFM or similarly encoded data) is generally written to an OPC area of the optical storage medium. This pattern data may be stored in random access memory (RAM), nonvolatile memory such as flash memory or electrically erasable and programmable read only memory (EEPROM), and/or other suitable electronic data storage (e.g., if OPC is performed more than once). Therefore, it is also desirable to minimize the amount of electronic data storage required to store pattern data (e.g., by taking advantage of the redundancy inherent in the encoded data).